In this study, a slant polariser design for directional antennas and antenna arrays is proposed. The polariser changes linear polarisation (vertical or horizontal polarisation) to slant (45 degrees) polarisation of a directional antenna and arrays. The polariser consists of five layers that are formed by metallic strips with different angular orientations on each layer. In this way wide impedance and pattern bandwidth is obtained. Theoretical background of the polariser is examined and calculated parameters for the polariser are given. The designed slant polariser is placed in front of the horn antennas and the stacked patch antenna array. The polariser has a wide bandwidth and operates in the frequency range of 6.5-18 GHz. The performance of these polarisers are examined with simulations and verified by experimental results. Measured radiation pattern results show that the polariser successfully changes the polarisation for directional antennas and antenna array. Also, measurement results show that by using the polariser these directional antennas and antenna arrays can radiate in both two linear polarisation components (both vertical and horizontal polarisation).